Not much remains from the original schematics.
Except a comperator is used as oscillator.

Most of these schematics on the web all seem to be the same for the oscillator- a LM311, some 100K resistors, as well one 47K, and 2x 10uF caps.

It does not work at all with the 10F200 comperator.

Which in turn goes to T1CKI of a larger PIC. I am setting the TMR1H to 0xff each interrupt, so I am getting 1:256 divider right now. Resulting in audible signals. Each time I toggle one I/O for a small piezo speaker.

Here the schematic. Only using 2 components, a resistor, and one capacitor.

I have tested 22uH and 680uH so far, both result in oscillation. That is about the range I'd be interested in.

Smaller inductors don't produce oscillation for instance one toroid seem to have only a few uH. But, they change the freq. of course if they are put in series with other inductors.

What to use as formula is not clear to me.

Seems to be I have to test a series of inductors, get the numeric frequency on a display, and apply calculus maths to get the formula.

Maybe it is the same as for the original circuit only in a different form.

No idea what the relay should be good for. Except it could be used to add Lx in series to a fixed inductor, and shorten it for calibration.

Still a lot of work until I get a numeric display, but I made it oscillating.

Most of the components are unneccessary however unfortunately it needs a stable voltage or the frequency will change. Maybe I'll use a dc/dc booster with an AA battery.

The basic approach, is after you get your oscillator working correctly is to write a frequency measurement routine. That frequency measurement can be the used to calculate the inductance.

That comparator-based oscillator is simple and works very well with high Q inductors, but huge errors occur when measuring inductors with high resistance. Let me know if you want Chris Krah's formula that shows the effects resistance has on frequency.

Good luck with the TTL based circuit, but why aren't you using an actual comparator? They are not expensive and it would be very hard to improve on the simple comparator based oscillator.

If you are going to measure inductance only, you won't need a relay; you can leave the resonating capacitor in the circuit all the time. For automatic zeroing, you can just store a constant to correct for small errors in the reference capacitor.

For a formula, if you are using the comparator oscillator, the inductance can be found with the classic formula below.

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Its basically multiple precision multiplication and division.

Appears that you have a lot of options to work through. Hope this helps at least a little.

The TTL oscillator was just an idea. I have built the circuit with a 10F200, and soldered a row connector to plug in carrier PCBs.

Floating point is not a problem even if the 16F74 does not have much memory. I hope at least.

Don't have comperators here at all, sure I can order from the UK and it arrives within 2 or 3 days. Except some 10F206, but they do not work with 100K resistors. Also have the original circuit on a carrier PCB.

Yes if you have that formula available? Most of the inductors I use have fairly low resistance, only some RF inductors here with 1000uH and a few 22000uH.

The relay was already added to the circuit board, however it is a problem, the frequency drifts even if I only turn on a LED.

Eventually I want to leave 22uH or maybe 10uH in place, and put the Lx in series. The circuit does not oscillate with smaller value inductors.